It was by chance that EL James, the creator of the publishing phenomenon of the year, Fifty Shades of Grey, began writing about sadomasochistic sex. While browsing in a bookshop in London some years ago, she picked up Macho Sluts, a collection of eye-wateringly explicit stories of dominatrixes and dungeons by American author Pat Califa.

"It was my first taste of something really hardcore, and I thought: this is interesting. After that, I read some more BDSM (bondage and discipline, dominance and submission and sadomasochism) and wondered: what would happen if someone from that world met somebody who didn't know anything about it?"

That idea was the kernel of the Fifty Shades trilogy, a sequence that has coined the salacious moniker "mommy porn".

Told from the perspective of student and virgin Anastasia Steele, the books follow the story of her seduction by an emotionally damaged, ridiculously handsome billionaire called Christian Grey. Grey is a skilled proponent of BDSM, and their romance unfurls against a backdrop of whips and handcuffs.

Initially spread by word of mouth, and helped by the discretion afforded by e-books, Fifty Shades was the fastest paperback to sell one million copies, with James becoming Amazon's biggest-selling author.

She has just won the Popular Fiction Book of the Year category at the UK's National Book Awards. By Christmas, an estimated 100 million people will have read it.

"It started to kick off last January; since then it has been mad, really crazy shakes," James says, jazz hands in the air to illustrate the intensity of the last 12 months.

She shelters behind thick, dark hair and a long fringe but it fails to disguise a deep tiredness. "I'm slightly shattered," she admits, clutching a large cappuccino. "I'm sorry if I'm not very awake. It's been a bit of a whirlwind."

In short James, real name Erika Mitchell, looks exactly what she is: a 49-year-old mother of two who has found herself catapulted to global superstardom. At one point this summer, the former TV producer was earning around €987,000 a week, while the film rights to the book sold for €3.8m.

It has not, however, been an easy ride. Her occasionally clunky prose has proved an easy target and the literary community has sniffed its disapproval.

"Journalists and critics think: 'why are people reading this and not my stuff?' I didn't set out to be Tolstoy. I wrote it for myself, for fun. The majority of people who read it love it – and quite a lot of people have read it," she smiles. "It made me feel slightly less of a pervert when other people enjoyed the fantasy as well..."

More hurtful have been the allegations that her books encourage domestic violence, despite the emphasis she places on consensuality.

"Nothing freaks me out more than people who say this is about domestic abuse," she says. "Bringing up my book in this context trivialises the issues, doing women who actually go through it a huge disservice. It also demonises loads of women who enjoy this lifestyle, and ignores the many, many women who tell me they've found the books sexually empowering."

James has been a reclusive figure on the media scene so far. Many of the "interviews" she has supposedly given have been no more than a summation of what she posts on her websites. She refuses to speak to most newspapers, fed up that she is always quoted out of context. (I underwent a 90-minute grilling by her agent before being granted access.)

By contrast, she delights in interacting with fans. Her Twitter feed shares her love of eating Nutella with a spoon, her fondness for walking her Westie dog, Max, near her home in Ealing, west London, and such domestic trivia as broken plumbing and an inability to eat a fish taco tidily.

It touches on her smoking – menthol slims, or "ghetto fags" – and her joy at settling down at the end of a day with a bottle of Oyster Bay sauvignon blanc and her Irish husband of 25 years, Niall.

Niall (53) is also a writer, primarily of screen drama, including Hornblower. Aside from the odd exuberant row, the two enjoy working at home together – in separate rooms. "Niall really understands the craft of writing. I don't like to publish anything unless he's read it. He's my absolute touchstone."

For his part, Niall is phlegmatic about having a wife famed the world over for her libidinous fiction – as well as one who earns hundreds of times more than he does. He is on record as saying that he is not Christian Grey, that he is the world's least romantic person, and that they don't have a "red room of pain" (what Grey calls his dungeon).

Has the stark contrast between her real-life husband and the dream man in her books caused any marital problems?

"It's probably why I wrote Christian," she says. "I could give it to Niall and say, 'hey, this is a manual . . .' No, Christian is fantasy. Niall is amazing, a lovely man who makes me laugh. Everything about Fifty Shades is fantasy: fantasy man, fantasy sex."

James began writing as a child in Buckinghamshire and had started various novels over the years. But it was the Twilight books by Stephenie Meyer that inspired her to write Fifty Shades as a steamy piece of fan fiction, a literary tribute to the vampire/virgin genre.

In late 2010, she started publishing the tale of Ana and Christian free on fanfiction.net, using the pseudonym Snowqueens Icedragon. It was so well received by fellow fans that in May 2011 she decided to publish Fifty Shades of Grey as an e-book. As Fifty Shades's success snowballed, selling more than 250,000 copies despite no publicity, she was approached by six big publishing houses, went with Vintage Books (part of Random House), and has never looked back.

In all that has followed, she has taken great care to keep her two teenage sons out of the picture.

"The boys haven't read the books. Why would they be interested in their mum's fantasies? They just want their mum."

If only it were that simple. James went to see her eldest in his school play recently. It was a Greek tragedy, and in one tableau all the females on stage were reading Fifty Shades. "I thought poor boy, bless his cotton socks."

But they get their own back by teasing her constantly. "They call me EL Haims," she says, adopting a thick Spanish accent. She also mentions, with some pride, that her eclectic musical taste – she writes to a playlist that includes everything from the Black Eyed Peas's 'Sexy' to Lakmé's 'Flower Duet' – is shared by her sons.

James's Chilean mother is a huge fan. Unperturbed by their racy nature, she has repeatedly read the trilogy in both English and Spanish. Her father died 10 years ago and she likes to think he would also have been proud of her, but she has "no idea" how he would have reacted.

As for friends, she laughs about the number of Facebook messages calling her a "dark, dark horse". Certainly, there is little about James's appearance and manner that hints at the vivid sexual imagination within. Nor is there much to indicate that she is a millionaire many times over, and, according to Time magazine one of the world's 100 most influential people.

Her domestic life is pretty normal, too. She spent the morning looking for socks and dealing with a broken dishwasher. Niall is in charge of the cooking but the laundry remains her domain.

They have promised themselves a smarter kitchen, but other than that James has yet to spend much of her new fortune.

She did buy some nice bras – but nothing with tassels. And now her millions look set to grow still further, with the launch of a new Fifty Shades bedroom accessory line.

Finally, in all the hullabaloo of 2012, is there a question that she has yet to be asked? She lights up.

"The difference between the way male and female fantasy is explored – it's interesting. Look at male fantasies: Lord of the Rings, Batman, The Avengers. It's lauded. Anything written by a woman, like Twilight, my huge inspiration, is derided. All female fantasy is derided. It's an insight into how misogynist the world is.

"Take the phrase 'mommy porn'. It's one of the most misogynist things I've ever heard in my life. It is derogatory!"

She bangs the table for emphasis. "How dare they? It's just a book, for god's sake. A love story in which people have sex – and they do do that, I seem to remember . . ."

She pauses for a moment.

"What the last year has done is made me slightly more confident about myself. I am trying to stop being so anxious about everything and enjoy living in the moment. Seize the day.

"I have to let it all wash over me, otherwise it is overwhelming. What I'd really like is to take two months off, put on my Ugg boots and write."

She knows fans are desperate for a book from Christian's point of view, but James's current plan is to rewrite two novels she began some time ago.

And – hold the front page – they may not contain the sort of sex for which she has become so famous. "It has to be the story of two people coming together. One of the books has no sex scenes at all."